Earth's 'oldest rocks' may not be that old

Published: Saturday, September 27, 2008, 12:16 [IST]

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London, September 27 : Researchers from the University of McGill in Canada, who had claimed to have found the oldest rocks on Earth, as old as 4.28 billion years, are now saying that the rocks may be 3.8 billion years old.

According to a report in new Scientist, These rocks, known as "faux-amphibolites", may be remnants of a portion of Earth's primordial crust - the first crust that formed at the surface of our planet.

The ancient rocks were found in Northern Quebec, along the Hudson's Bay coast, 40 km south of Inukjuak in an area known as the Nuvvuagittuq greenstone belt.

The discovery was made by Jonathan O'Neil, a Ph.D. candidate at McGill's Department of Earth and Planetary Sciences, and his team.

O'Neil and colleagues estimated the age of the rocks using isotopic dating, which analyzes the decay of the radioactive element neodymium-142 contained within them.

The dating method relies on the amount of the common isotope neodymium-142 in the rock. All rocks contain some neodymium-142, but rocks older than 4.2 billion years should contain more of it.

"You can precisely measure the amount of neodymium-142 and calculate a precise age for the rock," explained O'Neil. "In our case, it gave us an age of 4.28 billion years," he added.

That's significantly older than any rock yet found on Earth.

This could make the greenstone belt the oldest known rocks on Earth, just 300 million years younger than our solar system.

It also dates them close to the time when a massive object the size of Mars dealt Earth a glancing blow 4.53 billion years ago, knocking off the debris which formed the Moon.

However, the neodymium-142 levels may not be an indicator of the rock's age. O'Neil himself admits his team may instead be measuring the age of the magma from which the rocks formed.

"All rocks have precursor, something that came before they formed," said Martin Whitehouse of the Swedish Museum of Natural History.

O'Neil's team has also used a conventional method to date the rocks which suggests the greenstone belt is only 3.8 billion years old - about 200 million years younger than the current oldest rock - Acasta gneiss, which was discovered in Canada in 1999.

It is clear that O'Neil and his colleagues have discovered one of the oldest signals from the very early stages in our planet's development.

"But how the signal should be interpreted is going to be very controversial," said Whitehouse.

"On the weight of evidence from other studies in the area, I would still consider that 3.8 billion years is more likely the actual age of the rocks," said Simon Wilde of the Institute for Geoscience Research in Australia.